To the library world, he's an international leader - particularly in efforts to preserve books hundreds of years beyond their expected lifetime.

To a handful of people, he's a classmate from Schwab High School in Weatherly. He ranked second among 30 graduates in the Class of 1937.

And to Catholic Bishop Thomas Welsh of the Allentown Diocese, he's an older brother. He's 65. The bishop is 63.

He says they were very close in their growing up years in Weatherly until his brother went away to seminary at age 15.

The bishop in the previous post as Bishop of Arlington was not quite his bishop. He lives in Bethesda, close enough but not part of the Arlington Diocese.

William Welsh and his wife Winifred were in the Lehigh Valley over St. Patrick's Day weekend for a gathering at the bishop's house for a family celebration. "We're professional Irish persons," the librarian declared.

The front door of the bishop's South Whitehall residence had a message posted: "Blarney Spoken Here."

The group included a sister, Mrs. Mary Luchi, and her husband Andrew of St. Clair, Schuylkill County. He was sporting a button that said, "Kiss me, I'm Tyrolean."

Missing from the group was another brother, Edward Jr., of Redwood City, Calif., a retired mechanic for United Airlines.

Amid the partying at the back of the bishop's house, William Welsh took time out for an interview in therelative quiet of the front living room.

He holds a bachelor's degree in philosophy from the University of Notre Dame, Class of 1940. And he went on to law school there during 1940-41, only to drop out to go into the service.

It was at Notre Dame that he began his interest in library work by serving at an assistant librarian in the university library.

And he says that "one of the highlights of my life was when I got my honorary doctor of laws from Notre Dame." This was at Notre Dame's 139th commencement last May.

The citation said in part: "His imagination and good judgment earned him recognition in this country and abroad as an innovative leader in librarianship. Through his expertise the premier library of the land made influential strides in acquisition and cataloging practices.

"He has been farsighted and creative in recognizing the potential for modern technology in libraries, and his current experimentation in the preservation of library materials has worldwide implications."

The Library of Congress now has an annual budget of $242 million and a staff of 5,000.

It serves the world. Most of its holdings are in foreign languages.

It houses 80 million items, 20 million of them books. And the paper in about 25 percent of those books has deteriorated to the point where pages are brittle. The villain is acid in the paper.

Around the time of the Civil War, rags became scarce, so papermakers turned to wood pulp, which is not strong. They also switched from gelatin sizing to alum resin, which creates acid in paper in the presence of moisture in the air.

Welsh says that an answer for the Library of Congress is a mass de- acidification program - sucking the acid out of the books and then treating the pages with a zinc vapor that adds to their life anywhere from 400 to 600 years.

He says the Library of Congress is building a plant in Frederick, Md., that should be ready sometime in 1987 under this patented process. It is expected to treat a million books a year.

"For the moment, it will be just our books," Welsh says. But he says there are efforts being made to possibly lease the patent so other plants could be established to treat books for other institutions.

He says experimental work has been done with this process on 5,000 books. They were put in what might best be called a pressure cooker. They yielded 64 pounds of water. Then they were treated with the zinc gas, which leaves a residue of zinc carbonate to help preserve them.

The other major project is the optical disk. "It looks like a phonograph record. It's laser-produced and laser-read. It's relatively permanent.

"You can remotely store it, randomly access it and transmit it along telephone lines." Each disk can store up to 300 books.

Another approach is to develop the use of acid-free paper. "There are a couple mills that produce nothing but acid-free paper. But less than 1 percent of all paper is used for library materials," Welsh says.

Along with the joys of new technology, Welsh says he likes working with people. "I care about people. I try to keep their needs in mind.

"I like to get out and talk to people." The bigger the Library of Congress gets, however, the more difficult that becomes.